1. Field of the Invention
The present invention relates to a process for the preparation of metallic bottles. More particularly, the present invention relates to a process for preparing metallic bottles excellent in the adhesion, sealing property and pressure resistance of the seal and the durabilities of these properties by lap-bonding upper and lower members, each consisting of a formed cup of a metal, on the circumferential end portions thereof through an adhesive.
2. Description of the Prior Art
Bottle-like metallic vessels obtained by lap-bonding upper and lower members, each consisting of a cup-like article formed by drawing or draw-ironing of a metallic material, on the circumferential open end portions thereof to form a circumferential side seam are advantageous in various points over metallic vessels in the form of a can.
Conventional packaging metallic vessels include a so-called three-piece can obtained by double-seaming can lids to the top and bottom of a can body having a side seam to form sealed portions and a so-called two-piece can obtained by double-seaming a can lid to the open end portion of a cup-like can body formed by drawing or draw-ironing of a metallic material to form a sealed portion.
Metallic vessels having this double-seam structure are much limited in the pressure resistance of the sealed portion or the saving of a metallic material. More specifically, in a seam formed by double seaming, if a load is applied to the seam, the material constituting the seam is first deformed, and by this deformation, leakage from the seam or breakdown of the seam is caused under a relatively small load. In the field of packaging vessels, from the economical viewpoint and in order to reduce the weight of a vessel, it always is required to reduce the thickness of a metallic material. However, if the thickness of the body wall of a vessel is reduced, at the double-seaming step or at the step of preparing for the flanging operation, buckling is readily caused under a load applied in the axial direction of the vessel.
A metallic bottle obtained by lap-bonding the open end portions of upper and lower members, each consisting of a formed cup, is advantageous in that even if the thickness of a seam-constituting material is extremely small, deformation of the material is not caused on the seam and the seam can resist a load within the critical shear strength thereof irrespectively of the thickness of the material and that since the double-seaming operation need not be performed, the thickness of the side wall of the vessel can be reduced without any risk of occurrence of buckling.
However, in the case where a circumferential side seam is formed by lap-bonding the open end portions of upper and lower members through an adhesive, various problems should be solved so as to provide a strong bonding on the seam and give a reliable seal to the seam.
More specifically, in the lap bonding using an adhesive, so satisfactory bonding force can be obtained unless a certain pressure is applied to an adhesive layer interposed between portions to be bonded. In the case where a lap seam is straight as in an ordinary can body, compression of the lapped portion can be accomplished relatively easily, but when circumferential open end portions of formed cups are lap-bonded, it is very difficult to apply a pressure to the lapped portion.
As the method for applying a pressing force necessary for bonding to the lapped portion, there have heretofore been considered the following methods.
(1) A method in which the lapped portion is compressed by an external force, such as (a) a method in which upper and lower members are fitted together and the interior is compressed to outwardly expand the end edge to be located on the inner side of the seam or (b) a method in which a clamping force is given to the open end portion to be located on the inner side of the seam to reduce the diameter and the lapped portion is compressed by the elastic recovery force.
(2) A method in which the diameter of the open end portion is changed by thermal expansion to produce a pressing force to the lapped portion, such as a method in which the end edge located on the outer side of the seam is cooled and/or the end edge to be located on the inner side of the seam is heated.
(3) A method in which the difference of the dimension between both the end portions to be formed into the seam is utilized, such as a method in which the outer diameter of the open end portion to be located on the inner side of the seam is made larger than the inner diameter of the open end portion to be located on the outer side of the seam.
The method (1)-(a) is defective in that since the step of cooling and solidifying the adhesive under compression is necessary, the productivity is low and since the formed cup should be pressed in the axial direction so as to prevent divergence of the seam by the inner pressure, buckling deformation of the formed cup is readily caused.
The method (1)-(b) is disadvantageous in that since the open end portion to be located on the inner side of the seam should be pushed into a clamping tool, this open end portion is inconveniently deformed or the adhesive layer or primer applied to this open end portion is cracked or broken.
The method (2) is disadvantageous in that it is technically difficult to uniformly heat the end edge portion located on the inner side and if heating is possible, during the cooling and solidifying step, this inner end edge portion is contracted and a clearance is readily formed between this inner end edge portion and the adhesive layer. When the end edge portion located on the outer side is cooled, also the adhesive is cooled at the solidification temperature or a temperature close thereto, no satisfactory bonding force can be obtained. When the inner end edge portion is heated and the outer end edge portion is cooled, although the heat conductivity of the adhesive is about 1/1000 or less of the heat conductivity of the metal, no satisfactory temperature gradient can be obtained unless the thickness of the adhesive layer is sufficiently large, for example, at least 200 .mu.m. Moreover, this method is defective in that the surface of the metal is often heated excessively and therefore, the surface of the metal, especially the primer or adhesive layer, is readily deteriorated.
The method (3) is disadvantageous in that fitting of both the open end portions is difficult and if the thickness of the adhesive layer is sufficiently large, for example, 100 .mu.m or more, permanent deformation of the metallic material is caused in the seam or in the vicinity thereof.
In each of the foregoing methods, elastic deformation of the metallic material is caused in the seam, and a considerable strain is left in the adhesive. In this state, the adhesion failure is readily caused by a denting or impact force, and leakage is apt to take place with the lapse of time. Moreover, it is difficult to produce a uniform pressing force along the entire circumference of the circumferential end edge portion, and the thickness distribution of the adhesive layer or the bonding strength tends to become uneven.